Ultrasound Physics
Welcome
Ultrasound Basics
Vibration and Wave
Ultrasound Parameters
Medium Acoustic Property
Ultrasound Reflection
Ultrasound Refraction
Ultrasound Scattering
Ultrasound Attenuation
Ultrasound Application
Ultrasound Transducer
Piezoelectric Effect
Transducer Cosntruction
Array Transducer
Beamforming
Ultrasound Beamformation
Beam Focus
Beam Steering
Imaging
Pulse-echo Method
Imaging Method
Imaging Resolution
Ultrasound Imaging Artifacts
Signal and Circuit
Unipolar Transmitter
Bipolar Transitter
Transceiverg
Time Gain Control
Conditioning
Preprocessing and Postprocessing
Flow Dection
Doppler Effect
Continue Wave Doppler (CW)
Pulse wave Doppler(PW)
Color Flow Imaging
Safety
Intensity
Mechanical Index
Thermal Index
Cavitation
Regulations
Ultrasound Time-Gain-Compensation (TGC)
Ultrasound imaging is based on pulse-echo method. The transmitted acoustic pulse will lose its energy due to two main factors: beam spread and attenuation. As shown in Fig. 1, along the axial axis through the center of a piston transducer, the beam will reach its maximum at certain depth and then decrease monotonously. How fast it decreases is determined by the geometry and bandwidth of the transducer, and frequency and bandwidth of the excitation pulse.
The echo signal will decrease in the same way even if the tissue structure is the same. The gain for the echo has to increase with depth to maintain a uniform brightness of the image from near field to far field. Soft tissue has an attenuation of about 0.3dB/MHz/cm to the acoustic pulse. The Time-Gain-Compensation (TGC) has to compensate the lose from both of beam spread and attenuation.
The TGC amplifier usually is a variable gain amplifier with gain controlled by a TGC voltage curve. The curve can be generated by an analog oscillation circuit or a digital curve through DAC, triggered by the pulse transmit signal.
The most simple TGC curve is a saw-tooth curve with adjustable slope. For most imaging system, TGC curve is adjustable independently for each depth segments such as 1cm at 5MHz.